i don't get it. first u said that you want a rocket shoot out of a random point, and then u ask how can u make it to shoot out of the weapon. you're not making any sense.
well then, in the object editor of the unit you've chosen your model for, change the value of Projectile x, y and z (as i can remember) to the x, y and z values of the point that you want the missile to come out (check that in max).